How do you switch bike gears to gear 4 on Pokemon platinum?

In Pokémon Platinum, you don't actually switch bike gears like in a traditional cycling game. Instead, you have two bike settings: the Acro Bike and the Mach Bike. To switch between these bikes, you need to visit a bike shop in Eterna City and choose the bike that suits your needs, with the Mach Bike allowing for faster movement and the Acro Bike enabling tricks and stunts. Gear changes refer to different bike functionalities but are not explicitly numbered like in some other games.